This invention relates to apparatus for utilizing solar radiation to heat fluid. In particular, the invention pertains to improvements in the class of solar energy converters known as concentrators in which lenses are used to focus the radiant energy of the sun onto a heat absorbing and transferring surface.
Prior art solar energy concentrators may be generally characterized as having a conduit through which the fluid that is to be heated flows. The conduit is in an insulated housing, usually near its bottom. One or more lenses are arranged in a suitable support at a short distance from the conduit for focusing images of the sun on the conduit. The conduit is usually bonded to a metal plate which has a dark surface for enhancing absorption of stray solar radiation rays which are outside of the focal path of the lenses. Concentrating or focusing primary solar radiation which is collected over a large surface area onto a relatively smaller area on the fluid conduit enables the temperature of the fluid to be raised to a higher level than would otherwise be obtainable if the solar energy were distributed over a greater area.
In prior art solar energy concentrators, a substantial portion of the energy in the solar radiation spectrum which is converted to useful heat is lost to the ambient by reradiation, convection and conduction. The present invention is concerned with reducing these losses.
The principles and problems of prior solar energy concentrators are revealed in some prior patents. In U.S. Pat. No. 3,915,148, an array of Fresnel lenses is disposed in the top opening of a housing for concentrating or focusing primary radiation from the sun in parallel focal lines on an absorber comprised of several v-shaped conduits on the apexes of which the focal lines occur. A measure taken in the patent to reduce re-radiation of thermal energy from the absorber is to predetermine the angle between the walls of the v-shaped conduits so that one wall emits radiation to the other. There is nothing but air intervening between the absorbers and the lenses. The air convects and conducts heat away from the absorbers. It is in contact with the lenses which, in turn, have their outer surfaces in heat exchange relationship with atmospheric air. The flow of relatively cool atmospheric air over the outside of the lenses results in significant loss of heat from the interior of the concentrator by convection and conduction as well as by re-radiation.
U.S. Pat. No. 3,985,118 discloses a solar furnace which uses Fresnel lenses to focus solar radiation, gathered over a large area, onto a corresponding number of heat conducting members that extend into vessels which are occupied by the fluid to be heated. In this patent, a heat barrier or sheet is disposed between the lenses and the vessels on which the radiation is concentrated. The barrier has holes for passing the converging solar rays to the heat absorbing and conducting members. The heat barrier, however, absorbs secondary or re-radiated infrared radiation and does not participate in any substantial way in transferring heat to the fluid. The barrier would seem to reach thermal equilibrium.
Another solar energy concentrator for heating fluid is disclosed in U.S. Pat. No. 3,981,295. It is characterized by an insulating housing which has an array of lenses in its top and a pair of slightly spaced apart plates in its bottom. A thin film of heat absorbing fluid flows between the plates. The top plate on the side of the lenses is darkened to enhance absorption of radiation. There is nothing intervening between the absorber plate and the lenses to intercept secondary infrared radiation that would be emitted by the absorbing plate. The area of the absorber presented to the exterior of the housing is so great that a substantial amount of the heat could be re-radiated to the atmosphere. No means are provided for utilizing the re-radiated thermal energy.
U.S. Pat. No. 3,929,121 discloses a solar energy concentrator comprising an insulated housing with a cover in which there are a plurality of lenses. A number of glass bulbs are arranged in the bottom of the housing under the lenses for primary solar radiation to be focused on the bulbs. The bulbs have a dead air chamber disposed over a chamber which is occupied by the fluid to be heated. The last named chambers are connected in series to obtain a continuous fluid flow path. The dead air space has glass on one side of it and an absorbing surface on the other. The air in the space has some insulating quality but it will absorb no significant amount of infrared or heat radiation which is re-radiated by the absorber.
There are several other patents on solar heaters which use lenses to concentrate the radiation known to applicants but, on inspection, they appear to disclose no means for minimizing losses due to re-radiation of heat energy from the absorber. U.S. Pat. No. 1,672,750 simply has lenses focused on a broad area absorber plate under which water flows and there is only air space between the plate and lenses. U.S. Pat. No. 2,277,311 has a fluid conduit supported in a housing on insulating material with an array of lenses covering the coil. U.S. Pat. No. 1,683,266 discloses a solar heating apparatus using a parabolic reflector with the fluid conducting tube at the focus. An array of lenses across the parabola focuses onto reflectors on the parabolic surface which, in turn, redirect the focused beams to the tube. U.S. Pat. No. 3,822,692 also discloses a device that uses lenses to focus the sun's rays on water filled tubes for generating vapor to run an engine. U.S. Pat. No. 1,673,429 discloses a water heater comprising a plurality of lenses arranged along the path of a zig-zag shaped water tube within an insulating housing.